Communication systems wherein a wireless interface is provided for entities provided with a transmitter and/or receiver are known. The entities may comprise equipment such as mobile or fixed user equipment (e.g. a mobile telephone), a base station and/or other equipment provided with a transmitter and/or receiver. The communication via the wireless interface may comprise, for example, communication of voice, data, multimedia and so on.
A communication system typically operates in accordance with a given standard or specification which sets out what the various elements of the system are permitted to do and how that should be achieved. For example, the standard or specification may define if the user, or more precisely, user equipment or terminal is provided with a circuit switched service and/or a packet switched service. Communication protocols and/or parameters which shall be used for the connection may also be defined. The hierarchical order of various functions associated with a communications instance may also be defined. In other words, a specific set of “rules” on which the communication can be based on needs to be defined to enable communication by means of the system.
An example of the wireless systems is the public land mobile network (PLMN). A PLMN is a cellular system wherein a base transceiver station (BTS) or similar entity of a radio access network of the communication system serves user equipment (UE) such as mobile stations (MS) via a wireless interface between these entities. A more specific example of the so called second generation (2G) PLMN systems is the Global System for the Mobile communication (GSM).
A further development of the GSM is the so called Enhanced Data rates for GSM Evolution (EDGE). EDGE is a standard that has been prepared by the third generation partnership project (3GPP) and that is now also defined by the ETSI (European Telecommunications Standards Institute). A description of the GSM/EDGE Radio Access Network (GERAN) and channel coding for the GERAN can be found e.g. from 3GPP specification TS 45.003 v5.5.5 (2002-04).
The EDGE enables higher data rates than the more conventional 2G GSM. This improvement has been achieved, among other modifications, by changes in the modulation.
The GERAN is based on use of TDMA (Time Division multiple Access) transmissions. In TDMA based systems the transmission take place in time frames. Each frame can be divided into a plurality of slots. The division of the frames into the slots enables a plurality of users to share the frames. The TDMA frames can be seen as providing the physical channels of the communication media for the transfer of information between two nodes of the communication system. A slot can be used for consecutive frames to form a physical channel for the transmission. A burst is then transmitted within each slot.
A typical TDMA transmitter would comprise means for performing channel encoding, interleaving, burst formation, modulation, and the actual transmission, see FIG. 1. It shall be appreciated that these functions may be provided by means of separate entities or at least some of these functions could be provided by a functional block of the transmitter.
After the channel encoding the information bits are formed into entities known as blocks. The total number of bits in a block depends mainly on the selected encoder. A block is typically transmitted over several bursts, that is, over several consecutive frames, but in a slot.
The information bits in a block are spread to appropriate positions in the bursts by means of the interleaving. Typically the aim is to spread the consecutive information bits as far apart from each other as possible. In the above referenced technical specification 3GPP TS 45.003, a diagonal interleaver is given for handling blocks of 456 coded bits. A block of coded data is interleaver “block diagonal”, where a new block starts every 4th block and the data is distributed over 8 blocks. In the given interleaver:
                                                        for              ⁢                                                          ⁢              k                        =            0                    ,          1          ,          2          ,                      …            ⁢                                                  ⁢            455                          ⁢                                  ⁢                  b          =                      k            ⁢                                                  ⁢            mod            ⁢                                                  ⁢            8                                                                              j        =                              2            ⁢                          (                                                (                                      49                    ⁢                    k                                    )                                ⁢                                                                  ⁢                mod                ⁢                                                                  ⁢                57                            )                                +                      int            [                                          k                ⁢                                                                  ⁢                mod                ⁢                                                                  ⁢                8                            4                        ]                                              (        1        )                            where j is the position of the bit k within the burst b.        
If we name J the burst size (114 in the following example), K the block size (456), O the ordering parameter (49) and D the interleaving depth (8), the formula (1) can be written:
                                                                        for                ⁢                                                                  ⁢                k                            =              0                        ,            1            ,            2            ,                                          …                ⁢                                                                  ⁢                K                            -              1                                ⁢                                          ⁢          b                =                  k          ⁢                                          ⁢          mod          ⁢                                          ⁢          D                                                                              j          =                                    2              ⁢                              (                                                      (                                          O                      ×                      k                                        )                                    ⁢                                                                          ⁢                  mod                  ⁢                                                                          ⁢                                      J                    2                                                  )                                      +                          int              [                                                k                  ⁢                                                                          ⁢                  mod                  ⁢                                                                          ⁢                  D                                4                            ]                                      ⁢                                  ⁢                              Note            ⁢                                                  ⁢            that            ⁢                                                  ⁢                          K              D                                =                      J            2                                              (        2        )            
This formula is believed to work well as long as half of the block size (K/2) cannot be divided by the interleaving depth (D). This can be done as long as the block size K is larger than the interleaving depth D, i.e.:
                                          K            2                    ⁢                                          ⁢          mod          ⁢                                          ⁢          D                ≠        0                            (        3        )            
Formula (3) enables a check that the interleaving formula (2) works. In the above example the formula (3) would give ((456/2)mod 8)=4.
The communication systems typically have separated functions. As explained above, the functions can be divided hierarchically into various groups. These are often referred to as layers. Typically the lowest layer in a layer stack would comprise the actual physical transmission media, i.e. the logical traffic channels providing the radio bearers for transmissions. This is often referred to as the physical layer. Layer or layers on top of the physical layer contain functions such as radio link control, Medium Access Control (MAC: a sub-layer of radio interface layer 2 providing unacknowledged data transfer service on logical channels and access to transport channels). As only the physical layer is of interest in the context of understanding the present invention, the other layers will not be discussed in any further detail herein.
A Flexible Layer One (FLO) has been proposed for the GERAN within the third generation partnership project (3GPP) standardization. This is a new type of configurable physical layer for the GSM/EDGE Radio Access Network (GERAN). The advantage of the proposed new physical layer is that functions such as channel coding, interleaving and so on would then be specified at call setup stage. This would, in turn, mean that support of new services such as Internet Protocol (IP) Multimedia Subsystem (IMS) Services can be handled without having to specify new coding schemes. Furthermore this physical layer would be more in line to what is specified for the third generation (3G) Universal Mobile Telecommunications Service (UMTS) Terrestrial Radio Access Network (UTRAN).
The inventor has found that the existing diagonal interleavers as the one described above by may not be easily reused for this purpose and that in order to implement the proposal, new type of diagonal interleaver needs to be specified. A reason for this is that the proposal frees bits for transmission of information.
Thus there are some unsolved problems in this regard. Most importantly, the existing diagonal interleavers may no longer work in all situations, for example since the 3GPP proposal enables instances wherein half of the block size can be divided by the interleaving depth. That is, when the relation (3) above is not met, i.e.
            K      2        ⁢                  ⁢    mod    ⁢                  ⁢    D    =  0
the interleaving formula (2) does not work anymore. This becomes a problem since instead of the previous 57 information bits, the proposal enables transmission (and thus interleaving) of 58 bits and block size of 464 bits (=4 bursts). This would result to 464/2 mod8=0, i.e. the above referenced situation wherein condition (3) is not met.
This problem will be clarified by means of the following simple example of a case in which the relation (3) is not met. Lest assume that:                K=16 block size        J=4 burst size        O=1 ordering parameter        D=8 interleaving depth        
It is possible to check by (3) that indeed ((16/2)mod 8)=0, and consequently the condition (3) is not met. Table 1 shown in FIG. 4 lists the values given by the interleaving formula (2) for the above particular example.
As shown by table 1, from bit number 8 onwards, the interleaving formula (2) does not work without problems, since:                bit number 8 is mapped on the same position and the same burst as bit number 0;        bit number 9 is mapped on the same position and the same burst as bit number 1;        bit number 10 is mapped on the same position and the same burst as bit number 2;        bit number 11 is mapped on the same position and the same burst as bit number 3;        bit number 12 is mapped on the same position and the same burst as bit number 4;        bit number 13 is mapped on the same position and the same burst as bit number 5;        bit number 14 is mapped on the same position and the same burst as bit number 6; and        bit number 15 is mapped on the same position and the same burst as bit number 7.        
This could cause various problems in transmission and reception of the bits.